Different types of machines are used for dispensing small metered amounts of liquid for a variety of applications. In the assembly of surface mount printed circuit boards, one application is for dispensing many small dots of adhesive liquid on a circuit board for connecting components; another is for dispensing material over an area for encapsulating chips and/or for underfilling flip chips. Such dispensing machines are expected to run continuously to achieve high throughput, and are also expected to have a high degree of repeatability, i.e., to be able to dispense dots or areas with the same size within a very small tolerance.
Some systems for dispensing dots, including earlier designs, used bursts of high pressure on a container of liquid and had a separate valve for controlling flow. This type of system was improved upon with a system that used a positive rotary displacement pump instead of bursts of high pressure. In one model manufactured by Knight Tool Co., the assignee of the present invention, and distributed by CAM/ALOT under the name Camelot.RTM., an augering screw is housed in an augering chamber and is rotated to provide a carefully metered amount of liquid. A motor is coupled to the screw with a controllable electromagnetically operated clutch. The clutch has a top plate that is continuously rotated by the motor, and a bottom plate that is rotatably coupled to the augering screw through intermediate coupling members, including a metal bellows. The liquid to be dispensed is held in a cylindrical container, and is provided to the augering chamber under constant pressure of about 10 PSI, a pressure that is considered rather low.
To dispense liquid, a controller provides to the clutch a short, timed, electrical signal that induces magnetic attraction between the top and bottom plates. This attraction causes the plates to be engaged and to rotate together for a short period of time, thus causing the augering screw to rotate to dispense a small amount of liquid from the augering chamber through a nozzle. The controller also controls lead screw motors for moving the dispenser to a desired location along three mutually orthogonal axes.
Other systems that use a positive rotary displacement pump drive the pump with a stepper motor that has to be turned on and off frequently. Such motors, however, must be sufficiently durable so that they can start and stop many times without failure.
Dispensing system of this general type can be used to dispense an area of highly viscous liquid to cover the top of a semiconductor device for encapsulation, or to provide underfilling around and under a flip chip to provide thermal conductivity. When the liquid hardens over or around the device, it packages and protects the device. To cover the area, the dispenser can be moved in a selected pattern, such as a rastering pattern or a spiral pattern.
In such dispensing systems, the accuracy of the volume dispensed is critically important. The material used for encapsulation and underfill is filled with abrasive particles and has a very high viscosity, typically from 10.sup.5 to 10.sup.6 centipoise. Consequently, pressure of at least about 30 to 40 PSI is provided to the container that holds the liquid to overcome friction in the container and to dispense the liquid. If the pressure is insufficient to properly feed the augering screw, cavities of air can develop in the liquid, adversely affecting accuracy (this problem is known as "cavitation"). Because this pressure is relatively high compared to the dot dispensing systems, however, the liquid can bleed through the augering chamber and leak through the nozzle between dispensing cycles. Such leaked liquid can have a substantial detrimental effect on the accuracy.
To prevent such bleeding and leaking, some models of dispensers have a direct drive stepper motor with a reversible drive for drawing the liquid back after a dispensing cycle is completed. Reversing a motor quickly and frequently can adversely affect wear on the motor, however, and can also adversely affect accuracy.